There has been a working machine in which an operator thereof can set, at his/her will, an operation mode such as a power mode (P mode) which places emphasis on working efficiency and an economy mode (E mode) which places emphasis on fuel economy. On the other hand, a hybrid working machine including an electric motor for actuating a swing body, a traveling body and the like of the working machine has a structure in which an electric drive system and a hydraulic drive system are loaded together, the electric drive system driving the electric motor by electric energy, and the hydraulic drive system driving a work implement, a traveling unit and the like by actuating a hydraulic cylinder and a hydraulic motor by using hydraulic fluid for a hydraulic pump directly connected to an engine. Thus, when no output limit corresponding to the operation mode is imposed on the electric motor in spite of the output limit imposed on the engine in accordance with the operation mode being set, the operator who operates the hybrid working machine may perceive the difference in operability thereof as compared with the operability of the working machine, which is loaded only with the hydraulic drive system, as operational incongruity. Moreover, when no output limit corresponding to the operation mode is imposed on the electric motor in spite of the output limit imposed on the engine in accordance with the operation mode being set, there would be discordance generated between operating speeds of the electric drive system and the hydraulic drive system, which would also be perceived as incongruity by the operator who operates the hybrid working machine.
In order to eliminate such operational incongruity, there exists a hybrid working machine in which a torque limit is imposed on the electric motor according to the operation mode being set. For example, the torque limit corresponding to the operation mode is imposed on a swing electric motor which swings an upper swing body of a hybrid excavator. FIG. 7 illustrates a torque line graph of swing torque that can be exerted with respect to a swing speed in the hybrid excavator. A curve LP1 is a maximum torque line representing the maximum torque that can be output by the swing electric motor when the P mode is set. A curve LE1 represents a maximum torque line when the E mode is set. Under such torque limit, the maximum torque being output at the swing speed equal to or less than a predetermined swing speed is smaller when the E mode is set than when the P mode is set.